1. Field of the Invention
This invention relates to the field of integrated circuits. More particularly, this invention relates to integrated circuits having on-board diagnostic circuitry with which it is desired to communicate via a diagnostic interface.
2. Description of the Prior Art
It is known to provide integrated circuits with on-board diagnostic systems. Examples of such systems are BIST controllers for testing on-chip memories, boundary scan cell chains and more general scan cell chains. It is known to provide dedicated diagnostic circuit interfaces to integrated circuits, for example such interfaces as utilised by the TAP controllers of JTAG diagnostic systems built in accordance with the IEEE Standard 1149. The JTAG interface typically requires four to six external pins dedicated for this use on the integrated circuit package.
As integrated circuits increase in complexity, size and performance, there is a general requirement for an increase in the number of external pin connections that can be made to the integrated circuit to support its functional (non-diagnostic) operation. It is desirable that the on-board diagnostic systems of an integrated circuit should have a small impact upon the functional behaviour and performance of the integrated circuit.